1. Field of the Invention
The invention relates to a planar reference electrode for chemical sensors.
2. Description of Related Art
To measure ionic concentrations, for example ions such as Na.sup.+, K.sup.+ and Ca.sup.2+, or pH-value, potentiometric measuring methods are preferably used at the present time. For this purpose, in addition to an ion-selective sensor, a reference electrode is required to supply a constant electric potential during the measuring operation. So-called electrodes of the second type are usually employed to this end, for example silver/silver-chloride electrodes (Ag/AgCl-electrodes).
The Ag/AgCl-electrode is comprised of a silver wire that has been coated with a silver-chloride layer and which dips into an electrolytic solution having a constant chloride ionic concentration. The electrolytic solution is thereby in electrical contact via a diaphragm, for example porous ceramics or glass grinding, with the measuring solution (c.f., for example: K. Cammann, Das Arbeiten mit ionenselektiven Elektroden [Working with Ion-Selective Electrodes], 2nd edition, Springer Verlag, Berlin, Heidelberg 1977, pp. 44-47).
A reference electrode having such a complex structure, however, is not compatible with ion sensors and pH sensors, such as ChemFETs (Chemically Sensitive Field Effect Transistors); c.f.: P. Bergveld and A. Sibbald, Analytical and Biomedical Applications of Ion-Selective Field-Effect Transistors, Elsevier Science Publishers B.V., Amsterdam 1988, pp. 63-74 (G. Svehla (publisher), Comprehensive Analytical Chemistry, volume XXIII). Therefore, it has been proposed to cover a common ISFET (Ion-Sensitive Field Effect Transistor) with a layer of an uncharged gel (c.f.: J. Janata in J. Janata and R. J. Huber (publisher), Solid State Chemical Sensors, Academic Press Inc., Orlando 1985, pp. 101-103). The effect of the gel is slowing down the ISFET response by lengthening the diffusion path. Because of the decelerated ion diffusion in the gel and given a rapid change (&lt;1 min) from calibrating medium to measuring medium, the electric potential of the reference ChemFET remains constant for a short time (about 1 min) and can, thus, serve as a reference potential.
A further development of such a reference sensor is forming a hollow space across a pH-ISFET (pH-sensitive ISFET) situated on a silicon substrate. This hollow space is delimited by an anodically bonded glass layer (c.f.: Analyst, vol. 113 (1988), pp. 1029-1033), has a diameter of 500 .mu.m and a height of 200 .mu.m, and is filled with a hydrogel, i.e., a hydrophilic material. It may be that this pH-ISFET, which is "braked" with respect to response time, can--in combination with any ISFET--be used for a short time (change calibrating medium/measuring medium) as a reference sensor, but it is associated with some disadvantages. Thus, as far as manufacturing is concerned, its construction is coupled to the anodic bonding of a glass disk provided with holes on to a Si-wafer. On the one hand, this procedure is costly and, on the other hand, is not fully developed from a technological standpoint. Moreover, to achieve an adequate delay in the response characteristic of the "braked ChemFET" in such a configuration, very thick hydrogel layers must be applied to the ChemFET, namely layers having a thickness of about 200 .mu.m (in a direction perpendicular to the extensional direction of the substrate).